1. Field of the Invention
This invention relates to common mode noise filters, and more particularly, to a common mode noise suppression circuit applicable to perform broadband suppression with related to a common mode noise within a frequency scope of several GHz.
2. Description of Related Art
Since more and more complex electromagnetic environment in an electronic system, electromagnetic interference (EMI) phenomenon is much worse and becomes an obvious obstacle affecting regular operation of the system. Since rapid development of high speed digital circuits, data rate transmission is constantly increasing and researchers are driven to pay attention to suppress noise and crosstalk of digital system. Ideally, a differential signal may maintain well original signal aspect and maintain low electromagnetic radiation or electromagnetic interference. However, in an actual circuit, unbalanced delay and amplitude, or unbalanced design of input/output register or package layout may cause the differential signal to generate different rising/falling edge time such that unwanted common mode noise attaches the differential signal. With respect to high speed data transmission interface, for instance, Serial ATA, PCI-E, OC-192, Gigabit Ethernet, etc., a cable is always needed to transmit the differential signals between different electronic devices. At this time, a common mode noise may be coupled to an input/output cable and is formed to be an excitation source such that the input/output cable becomes an EMI antenna. Hence, in order to solve electromagnetic interference problem of the input/output cable, it should suppress common mode noise on a differential signal route so as not to affect quality of the differential signal.
With regards to this, some methods for suppressing common mode noise of differential signals are provided, wherein a common mode choke is the most typical one. The common mode choke consists of two independent coils with the same amount of wire loops winding the same magnet, wherein its structure equals to a winding or feed through core coil, and it may generate high conductive impedance for common mode noise and generate impedance approaching to zero for differential signal via high magnetic conductivity needle by summation and subtraction of self inductance and mutual inductance. However, the common mode choke works only with MHz scope, the common mode choke is not easily designed to be applied above high frequency section of GHz because of frequency characteristic and parasitics of ferromagnetic material, and manufacturing process and complex structure of the common mode choke is not suitable to modern miniaturized circuits. Furthermore, there provides a small common mode suppression filter which could work within MHz scope according to magnetic flux elimination principle and adopts Low Temperature Co-fired Ceramic (LTCC) technology. Additionally, there also provides some common mode suppression filters using pattern ground structure to eliminate common mode noise, wherein it may have broadband common mode suppression effect within GHz scope and have low cost. However, dimension of the pattern ground structure is half or quarter of wavelength of transmission signal, it would occupy large ground area of the circuit board such that area of made common mode filter fails to be reduced.
Therefore, it is imperative to how to provide a common mode noise suppression circuit which may solve various disadvantage of prior arts for improving shortcomings of prior arts having too high area and bad operation performance at high frequency.